Lawyers credited revelations from a "love triangle" for producing a rare conviction for insider trading yesterday, as the former Belgo boss Tim Power admitted leaking price-sensitive information about the sale of the restaurant chain.

The one-time multimillionaire pleaded guilty to insider trading after first flouting City rules in 1997. At the time he was operations manager of the mussels-to-frites chain and married to Chantal Brenninkmeyer, an heiress to the Dutch dynasty behind the C&A store chain.

A few months later, following Belgo's £9.8m reverse takeover by cash shell Lonsdale Holdings, he breached City regulations again. London's Southwark crown court heard that not long afterwards his wife's lover "blew the whistle" on him.

That led to the start of a lengthy investigation by the then Department of Trade and Industry (DTI) - now the Department for Business, Enterprise and Regulatory Reform - that included covertly recording some of Power's conversations.

The 43-year-old is expected to be sentenced early next month, though the judge has told him he will not be jailed.

Insider trading is rife in the City though notoriously difficult to prosecute. The Financial Services Authority has studied announcements by FTSE 350 companies and reckons insider dealing was linked to 22% of them. There is likely to be unease among City regulators that Power will not face a prison sentence though he has served more than 163 days in custody.

The court heard both his insider breaches involved confidential information only "those on the inside" could have known about at the time.

Prosecutors said Power first broke the law when he revealed Lonsdale was to be Belgo's new owner. When the deal became public knowledge Lonsdale's shares quickly rose in value.

A few months later Belgo plotted a takeover of three upmarket London restaurants - Daphne, The Collection and Pasha. Once again, Power secretly disclosed what was going on. Release of the news saw Belgo's share value climb.

The identity of the man who caused Power's downfall was revealed during pre-trial legal argument last week. The lawyer Simon Williams, at the time an investigator for the DTI, was asked how Power's name first emerged during investigations into Belgo and Lonsdale share dealings.

He replied: "There was a gentleman ... who blew the whistle to the Stock Exchange about activities he alleged occurred ... he mentioned Mr Power."

Williams was then asked whether he had been aware of a relationship. He responded: "From recollection, I think there was someone called Chantal Brenninkmeyer who had been married to Mr Power and had some association ... a love triangle, or whatever you want to call it."